v0.15.0 - new APIs to remove local overrides

Added a few new APIs to help removing local overrides for better testing experiences:

# If the user_id is None, the override for all users for the gate/config/experiment will be removed 

def remove_gate_override(gate: str, user_id: Optional[str] = None)

def remove_config_override(config: str, user_id: Optional[str] = None)

def remove_experiment_override(experiment: str, user_id: Optional[str] = None)

def remove_all_overrides()